Hibernating ground squirrels have dramatically decreased heart rates (3-5 beats per minute) and blood flow, which should put them at risk of forming blood clots. In response, they have several adaptations during hibernation that prevent blood clotting, including 3-fold decreases in Factors VIII (FVIII) and IX (FIX), and 10-fold decreases in von Willebrand factor (vWF), neutrophils, and platelets. The specific aims of this proposal are to study the effects of hibernation on platelet activity, formation of deep vein thrombi (DVT), and activation of fibrinolysis. By implanting ground squirrels with telemetry sensors, we will be able to determine when animals are in the five stages of hibernation: non-hibernating, entering hibernation, hibernating, interbout arousal, and emerging from hibernation. During hibernation, platelets are stored in the spleen at temperatures of 4-80C for up to six months. Within two hours of arousal in the spring, platelet levels return to normal and do not appear to be cleared rapidly. In contrast, human platelets that are stored in the cold are rapidly cleared by the liver when re-injected back into a patient. Ground squirrels will be transfused with platelets from non-hibernating and recently aroused animals, as well as platelets stored at warm or cold temperatures, to determine their rate and location of clearance in vivo. Extracellular traps, or DNA nets, from neutrophils have been detected in DVT, suggesting a protective role in the decrease of neutrophils as well. The activity of platelets and neutrophils isolated at all five stages of the hibernation cycle will be measured. These results could have direct application in extending storage of human platelets in the cold and preventing DVT. Finally, hibernation appears to increase fibrinolysis, which could help break down any clots that do form under low flow conditions. Understanding of how a ground squirrel can adapt to extreme physiological stresses on their cardiovascular system may lead to medical advances to prevent DVT, store human platelets in the cold, and regulate blood coagulation in cases of accidental or induced hypothermia. PUBLIC HEALTH RELEVANCE: This research may lead to medical advances to prevent blood clots, store human platelets in the cold, regulate blood clotting in cases of accidental or induced hypothermia, and treat hemophilia B or von Willebrand disease.